Objective: According to in vitro data, the polymorphic cytochrome P450 enzyme 2C9 (CYP2C9) may be the major S-ibuprofen hydroxylase. In humans, there are 2 variants of CYP2C9 with a high population frequency. We studied their impact on ibuprofen pharmacokinetics and on the inhibition of cyclooxygenases 1 and 2.
Methods: Kinetics of an oral dose of 600 mg racemic ibuprofen were studied in 21 healthy volunteers with all combinations of the CYP2C9 variants *2 (arginine144cysteine) and *3 (isoleucine359leucine). Blood concentrations of racemic ibuprofen and of S-(+)-ibuprofen and R-(-)-ibuprofen were measured by HPLC, and thromboxane B(2) and prostaglandin E(2) were measured with use of an enzyme immunoassay. Data were evaluated with a population pharmacokinetic model that integrated pharmacogenetic information.
Results: The pharmacokinetics of racemic and of S-ibuprofen depended on the CYP2C9 isoleucine359leucine amino acid polymorphism: population mean S-ibuprofen clearances were 3.25 L/h (95% confidence interval [CI], 2.84 to 3.73), 2.38 L/h (95% CI, 2.09 to 2.73), and 1.52 L/h (95% CI, 1.33 to 1.74) in carriers of the CYP2C9 genotypes *1/*1, *1/*3, and *3/*3, respectively. The CYP2C9 variant *2 exhibited no significant effect. Ex vivo formation of thromboxane B(2), reflecting cyclooxygenase type 1 inhibition, depended significantly on the CYP2C9 polymorphism. The maximal inhibition of thromboxane B(2) formation and the area under the effect-time curve were larger in carriers of the slow CYP2C9 genotypes *1/*3, *2/*3, and *3/*3 than in *1/*1 carriers; the same trend was observed for prostaglandin E(2), reflecting cyclooxygenase type 2 inhibition.
Conclusions: The reduced S-ibuprofen total clearance accompanied by increased pharmacodynamic activity may have medical impact in patients receiving ibuprofen.